4—Lakes

OVERVIEW

The fact that lakes occupy such a small fraction of the landscape belies their importance as environmental systems and resources for human use. They are major recreational attractions for Americans. Sport fishing, swimming, and boating are highly popular pastimes, and lake-front property has a high economic value. Large lakes and reservoirs are used as drinking water supplies; the Great Lakes alone serve as the domestic water supply for approximately 24 million Americans, and many more Americans rely on man-made reservoirs and smaller lakes for their source of drinking water. Lakes are used by humans for many commercial purposes, including fishing, transportation, irrigation, industrial water supplies, and receiving waters for wastewater effluents. Aside from their importance for human use, lakes have intrinsic ecological and environmental values. They moderate temperatures and affect the climate of the surrounding land. They store water, thereby helping to regulate stream flow; recharge ground water aquifers; and moderate droughts. They provide habitat to aquatic and semiaquatic plants and animals, which in turn provide food for many terrestrial animals; and they add to the diversity of the landscape.

The myriad ways in which humans use lakes, along with the numerous pollutant-generating activities of society, have stressed lake ecosystems in diverse ways, frequently causing impairment of lake quality for other human uses. Stresses to lakes arise from easily

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4—
Lakes
OVERVIEW
The fact that lakes occupy such a small fraction of the landscape belies their importance as environmental systems and resources for human use. They are major recreational attractions for Americans. Sport fishing, swimming, and boating are highly popular pastimes, and lake-front property has a high economic value. Large lakes and reservoirs are used as drinking water supplies; the Great Lakes alone serve as the domestic water supply for approximately 24 million Americans, and many more Americans rely on man-made reservoirs and smaller lakes for their source of drinking water. Lakes are used by humans for many commercial purposes, including fishing, transportation, irrigation, industrial water supplies, and receiving waters for wastewater effluents. Aside from their importance for human use, lakes have intrinsic ecological and environmental values. They moderate temperatures and affect the climate of the surrounding land. They store water, thereby helping to regulate stream flow; recharge ground water aquifers; and moderate droughts. They provide habitat to aquatic and semiaquatic plants and animals, which in turn provide food for many terrestrial animals; and they add to the diversity of the landscape.
The myriad ways in which humans use lakes, along with the numerous pollutant-generating activities of society, have stressed lake ecosystems in diverse ways, frequently causing impairment of lake quality for other human uses. Stresses to lakes arise from easily

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identifiable point sources such as municipal and industrial wastewater, from nonpoint degradation, from urban and agricultural runoff within a lake's watershed, and from more insidious long-range atmospheric transport of contaminants. Major categories of stresses include excessive eutrophication from nutrient and organic matter loadings; siltation from inadequate erosion control in agricultural, construction, logging, and mining activities; introduction of exotic species; acidification from atmospheric sources and acid mine drainage; and contamination by toxic (or potentially toxic) metals such as mercury and organic compounds such as polychlorinated biphenyls (PCBs) and pesticides. In addition, physical changes at the land-lake interface (e.g., draining of riparian wetlands) and hydrologic manipulations (e.g., damming outlets to stabilize water levels) also have major impacts on the structure and functioning of lake ecosystems.
No lake in the United States is entirely free from such stresses, but the stresses are not always severe enough to impair lake ecosystems or their usefulness for human activities. Nonetheless, thousands of U.S. lakes (and reservoirs) covering several million acres of water surface have become degraded to the extent that some type of activity is necessary to make them more usable resources and ecosystems.
Lake restoration is a relatively recent activity. Historically, the term restoration has been applied broadly in lake management to an array of actions aimed at improving lake conditions for designated human uses (e.g., contact recreation, fishing, water supply). Return of a lake to its pristine condition has not been an explicit goal of most lake restoration projects, although these actions often improve some aspects of a lake's ecological attributes. As such, most so-called lake restoration projects are actually rehabilitation efforts (in the sense of the definitions in Chapter 1), and many are merely designed to manage (mitigate) undesirable consequences of human perturbations. For reasons of historical precedence, a broader definition of the term restoration is used in this chapter, but a distinction is made between methods that improve ecosystem structure and function (restoration in the broad sense) and methods that merely manage the symptoms of stress. Lake restoration began in the United States about 20 years ago, primarily in response to problems of nutrient overenrichment. A lake improvement program, the Clean Lakes Program was established in 1975 within the U.S. Environmental Protection Agency by Section 314 of the 1972 Federal Water Pollution Control Act Amendments (P.L. 92-500). Between 1975 and 1985, federal funds were provided for Clean Lakes projects on 313 lakes in 47 states and Puerto Rico; 87 percent of the Clean Lakes funds have been used for lake improvement projects (U.S. EPA, 1985). Matching state and/or local

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funds typically are involved in these projects, and several states with large numbers of lakes have developed their own programs. As problems of lake acidification became more widely recognized during the past decade, restoration of acidified lakes by addition of limestone has become a relatively common practice in some northeastern states, as well as in Scandinavia.
For long-term restoration, it is essential to control the source of the problem. In the case of eutrophication, this means decreasing the loading of nutrients, particularly phosphorus, from various watershed sources. In some cases, this also means that loadings of silt and organic matter must be decreased. Control of external sources is sufficient to return some lakes to their former conditions, but in many cases the changes in the lake have been so dramatic—major shifts in biota, loss of habitat, physical changes in bottom sediments, and lake hydrology—that merely turning off the loadings is not sufficient to improve water quality and ecosystem structure, at least in a reasonable time frame. In-lake restoration techniques must be employed.
Numerous methods have been developed to restore lakes or improve their condition; this chapter describes more than 25 such methods. Available methods range widely in effectiveness, cost, frequency of use, and range of applicability. For example, methods that require addition of chemical agents to lake water are limited to small-and medium-sized lakes for economic reasons. Methods that use biological agents are potentially effective at low cost even in large systems because of low initial costs and the absence of labor and maintenance expenses. Many methods are applicable only to a single type of problem (e.g., liming to mitigate acidification). Others are potentially useful in restoring lakes degraded by a range of stresses; for example, dredging may be used for siltation, nutrient buildup, and toxic contaminant problems. Because eutrophication is the most widespread and longest-studied lake problem, more methods have been developed to restore eutrophic lakes than to address all other problems put together. Aside from removing contaminated sediments by dredging or covering them with uncontaminated sediment, few methods are available to restore lakes degraded by toxic substances.
Our ability to assess the effectiveness of past lake restoration projects and to compare the effectiveness of different restoration methods is severely limited by three factors. First, and perhaps most important, surveillance of lake conditions for an adequate period of time before and after a restoration attempt has been done on relatively few lakes. In some cases, sufficient surveillance probably was done, but rigorous analysis and interpretation of the data were not a part of the surveillance effort. All too often the data are not readily

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available for others to assess. Second, lake restoration projects usually are considered to be operational activities rather than research and development projects, and as a result they are designed to produce the desired effect—a restored lake—by whatever combination of methods seems likely to succeed. It usually is not possible to determine which of several techniques used simultaneously on a lake actually produced the measured improvements, even if detailed monitoring is done. Third, the goals of restoration projects are not always clearly defined, and it is difficult to judge the degree of success when clear objectives have not been set.
The above comments notwithstanding, many successful lake "restorations" have been documented, starting perhaps with the widely publicized case of Lake Washington, a large, deep lake in Seattle that was becoming increasingly eutrophic from municipal sewage effluent and was restored in the mid-1960s by diverting the effluent from the lake. Success in this and other cases generally has been defined in terms of restoring an aquatic resource for some human activities rather than restoring an ecosystem to its original condition. It is often assumed that improvements that benefit human uses of lakes lead to an improvement in the lake's ecology. There is no basis to assume, however, that water quality enhancements such as improved water clarity actually restore lake ecosystems to their original (presettlement) conditions. Restoration failures are less widely publicized, of course, but several cases have been described in which a project produced fewer improvements than anticipated in lake quality (see Appendix A). Analysis of these failures is important because we can learn as much about the factors leading to successful restoration from such projects as we can from success stories.
Lake restoration projects typically focus on restoring only one part (the lake) of a connected stream-wetland-lake system within a watershed. When wetlands are considered at all in lake restoration projects, it is typically for diversion of nutrient-laden storm water runoff or sewage effluent into the wetland in an effort to obtain nutrient uptake by wetland vegetation. Such diversions may provide a temporary lowering of nutrient loadings to lakes, but wetland flushing during high flow periods may result in little net annual retention of nutrients by the wetlands. The impacts of diversion on wetland ecology generally are not taken into account in deciding whether to proceed with such projects.
Although many techniques are potentially available to restore degraded lakes, the science of lake restoration is inexact, and the outcome of applying a given technique to a particular lake is difficult to predict accurately. Lake restoration technology can be advanced by

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ensuring that projects are monitored adequately so that the effects of various manipulations can be assessed properly. In this context, a lake restoration project should be considered as part of a long-term, ongoing management program rather than a one-time, permanent solution to a lake's problems.
INTRODUCTION-IMPORTANCE OF LAKES
Humans have always been attracted to lakes. Human settlement on lakeshores can be explained by practical reasons—lakes provide food and drinking water and a convenient means for personal transport and conveyance of goods—but can there be any doubt that even the Neolithic Swiss lake dwellers enjoyed their homes partly because of the beauty of their surroundings? Today, we prize small inland lakes especially for their recreational assets, including their visual appeal and the feeling of being close to nature that a "day at the lake" provides. Fishing, swimming, and boating are highly popular pastimes throughout the United States. Recreational fishing on inland lakes is estimated to generate more than $1.3 billion (1985 dollars) in economic activity annually in the state of Minnesota alone (Minnesota Department of Natural Resources, Office of Public Information, unpublished data, 1990), and comparable figures can be cited for many other states. In urban areas, lakefront homes are in high demand and command premium price tags; lakefront property in rural areas has a high commercial value for development of vacation homes. All too often, the attributes that give rise to a lake's recreational value—clear, high-quality water; scenic shorelines; prized game fish—are impaired by developments that were stimulated by the presence of these values.
There are about 100,000 lakes with areas greater than 40 hectares (1 ha = 2.47 acres) in the conterminous United States (Duda et al., 1987). Although natural lakes are found in most of the 50 states, they are especially common in several regions, owing to specific geological conditions: in the Upper Midwest, New England, New York, and Alaska, as a result of glacial activity; in Florida, where most lake basins are the result of chemical dissolution of underlying limestone; along major rivers like the Mississippi, where channel meandering has formed lake basins; and in mountainous areas of the Far West, where glaciers and volcanic activity have produced most of the lakes. In regions where natural lakes are rare or absent, artificial lakes (reservoirs) have been developed by damming rivers and streams to provide the benefits (e.g., recreation, water supply, water storage capacity) that natural lakes provide elsewhere.

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Large lakes and reservoirs are used as public water supplies; the American Water Works Association (Achtermann, 1989) estimates that 68 percent of the water used for domestic purposes by the 600 largest utilities (>50,000 customers) comes from impounded surface waters (natural lakes and man-made reservoirs). For simplicity, in this chapter the term lake refers both to natural impoundments and to man-made reservoirs. The five Great Lakes alone supply domestic water to some 24 million Americans. Lakes provide many other economic benefits to society and are used for such diverse purposes as commercial fishing, transportation, irrigation, and dilution of wastewater effluents. Not all of these uses are compatible. The use of lakes as receptacles for wastewater obviously is likely to impair their usefulness as water supplies and recreational resources, but more subtle incompatibilities also exist. For example, the production of warmwater game fish is enhanced by increasing nutrient levels, at least up to a point, but swimmers prefer water to be as clear (hence, unproductive) as possible.
STRESSES ON LAKES
Classes of Stresses and Their Effects
Lake ecosystems are subject to stress from a wide range of human activities within their watersheds and along their shorelines and from the variety of ways that humans use them. These stresses often have caused significant impairment of lake quality. Six major classes of stresses have been important in degrading the quality of U.S. lakes in recent decades:
excessive inputs of nutrient and organic matter, leading to eutrophication;
hydrologic and physical changes such as water-level stabilization;
siltation from inadequate erosion control in agricultural and mining activities;
introduction of exotic species;
acidification from atmospheric sources and acid mine drainage; and
contamination by toxic (or potentially toxic) metals such as mercury and organic compounds such as PCBs and pesticides.
In addition, chemical stresses to lakes can be categorized according to source as (1) point sources (such as municipal wastewater), which generally are the easiest to identify and control; (2) nonpoint or diffuse

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sources such as urban and agricultural runoff from a lake's watershed; and (3) long-range atmospheric transport of contaminants (the most difficult to measure and control). These stresses result in a variety of impacts on lake quality relative to human use and ecological integrity.
The specific impacts of stresses on lake ecosystems depend on the nature of the stress and the characteristics of the lake, but some responses are common to several categories of stress. For example, stress-impacted lakes tend to lose sensitive native species. Their replacement by stress-tolerant native or exotic species often does not fully compensate for the loss and leads to lower biodiversity and simplified food webs. Many types of stress result in loss of habitat; often this is the proximate cause of species losses. Many kinds of stress produce ''nuisance conditions," that is, proliferation of a native or exotic organism or deterioration in a physical-chemical property (such as water clarity) to the extent that beneficial uses of the lake are impaired. Finally, the development of toxic levels of contaminants in biota results not only from direct loading of toxic materials to lakes but also from indirect effects of other stresses (e.g., solubilization of aluminum as pH is decreased by acid deposition).
EUTROPHICATION
Of the six categories of stress, problems related to nutrient overenrichment and excessive plant production are probably the most common and have received public and scientific attention for the longest time. Concern about lake eutrophication from municipal wastewater extends back at least to the 1940s and the classic studies of Sawyer (1947) on the relationship between springtime concentrations of inorganic phosphorus and nitrogen and the occurrence of algal blooms in summer. By the 1960s, widespread concern existed about increasing eutrophication of the Great Lakes, and nutrient enrichment problems were recognized in numerous inland lakes. A large-scale research program funded primarily by federal agencies was undertaken on eutrophication in the 1960s and 1970s. This program led to improved understanding of the extent of the problem in U.S. lakes, delineated specific causes of the problem in some lakes, generated quantitative relationships between rates of nutrient loadings (especially of phosphorus) to lakes and water column responses in the lakes, and developed techniques to restore lakes degraded by eutrophication.
Eutrophication results in numerous ecological and water quality changes in lakes. The chain of events leading to use impairment is

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roughly as follows. Increased input of nutrients, especially phosphorus, leads to an increased incidence of nuisance blooms of algae (especially blue-green algae), leading to a loss of water clarity, a buildup of organic and nutrient-rich sediments, loss of oxygen from the bottom waters of the lake (which in turn, accelerates nutrient recycling processes), and changes in the lake's food web structure. Secondary nutrient limitation by silica or nitrogen that results when phosphorus levels are elevated also leads to changes in the phytoplankton community and to the development of nuisance species of algae (e.g., blue-green forms). Proliferation of macrophytes is also associated with eutrophication, especially in shallow lakes, but these problems are not tied directly to excessive rates of nutrient loading (see "Exotic Species," below). Although increases in nutrient levels enhance fish production, the loss of habitat (e.g., by sediment buildup, deoxygenation, undesirable proliferation of macrophytes) and food sources (by food web simplification) causes a shift from more desirable game fish to less desirable species, especially in more extreme cases of eutrophication. Stocking of exotics and overfishing exacerbate this problem. From a human use perspective these changes create numerous problems, including the following: fouling of boats and structures (by algal growths), loss of aesthetic appeal, accessibility problems for swimmers and boaters (because of macrophyte proliferation), economic damage to resort and property owners, and increased costs and technical difficulties of treating water for drinking purposes (because of taste and odor problems and increased potential for trihalomethane production).
The causes of eutrophication resulting from human activity are reasonably well understood. Once an oligotrophic lake has been made eutrophic, processes develop that may delay recovery after nutrient loadings have been decreased. If the hypolimnion becomes anoxic, recycling of phosphorus from the sediments is enhanced, in effect increasing the efficiency of use of the phosphorus input. During the eutrophic phase many changes may occur that will not be automatically reversed by a reduction in nutrient supply, such as loss of desirable macrophyte, invertebrate, and fish species. Nutrient reduction is a necessary, but not always a sufficient, condition for reversal of eutrophy.
Point sources of nutrients are the primary cause of excessive loadings in some lakes, but nonpoint sources (urban and agricultural runoff) contribute most of the nutrient input to the majority of U.S. lakes. Based on a modeling exercise with loading data on phosphorus for 255 lakes in the eastern United States, Gakstatter et al. (1978) concluded that only 18 to 22 percent of the lakes would show a

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measurable improvement in trophic conditions (which they assumed would require at least a 25 percent reduction in phosphorus inputs) if an effluent standard of 1 mg of phosphorus per liter were imposed on municipal wastewater treatment plants. Only 28 percent of the lakes would show measurable improvement if all their point sources of phosphorus were removed. Thus, most of the lakes (72 to 82 percent) in this analysis would require control of nonpoint sources of nutrients to achieve measurable improvements in trophic conditions.
HYDROLOGICAL AND RELATED PHYSICAL CHANGES
The watersheds of lakes in urban and agricultural areas clearly are no longer ecologically the same as they were in presettlement days, and such land use changes are a primary cause of the stresses described in this section. What is not so widely recognized is the fact that important physical properties of lakes themselves, such as water residence time, water level, and basin morphology, are often modified significantly in developed areas. In turn, these changes can have untoward effects on water quality and ecological conditions. The importance of morphology in determining a lake's basic level of productivity is a fundamental concept in limnology.
Diversion of stream flow into lakes to provide water for urban or agricultural uses outside the watershed has occurred in some western states; Mono Lake, California is probably the best known example. The resulting decline in water supply to the lake has caused long-term lowering of the lake level, an increase in the lake's salinity, and ecological damage to tributary streams and to the lake itself (NRC, 1987). A much more widespread practice nationwide is the stabilization of lake levels by regulating outflows with a control structure (dam) at the lake outlet. This practice minimizes flooding of shoreline developments during wet periods and prevents loss of access to the lake due to receding shorelines during dry periods. However, long-term water-level stabilization also leads to loss of ephemeral wetlands in nearshore areas, converting them either to permanently dry upland areas or to lake littoral area. Fluctuating water levels are thought to have a cleansing effect on littoral sediments (oxidizing organic deposits); accumulation of such deposits in nearshore areas of lakes with stabilized water levels contributes to poor water quality and loss of fish spawning areas.
Changes in water level also affect fish reproduction directly by regulating access to spawning areas in the littoral zone, streams, or surrounding wetlands. Consequently, coordination between agencies that regulate water level and agencies that manage fisheries can

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have significant benefits. For example, the level of Lake Mendota, Wisconsin, had generally been lowered in winter to protect shoreline structures from ice damage. As a consequence, northern pike were prevented from spawning in the marshes around the lake. This problem was recognized in 1987. Beginning in 1988, the water level was raised about 15 cm during the spawning season (late March to early April). Numbers of spawning northern pike increased about two-fold in 1988 and about eightfold in 1989 (Johnson et al., 1992). There has been no increase in the incidence of ice damage to shoreline structures.
Water residence times of lakes in developed areas are affected by water-level stabilization, as well as by diversion of streams into or out of a lake's drainage basin (thus also affecting watershed size and loading rates of nutrients and pollutants). Lake Okeechobee, Florida, is an extreme case of human-induced changes in lake morphometry, watershed area, water level, and other hydrologic characteristics that resulted in a variety of water quality problems (see Kissimmee River case study, Appendix A).
SEDIMENTATION
Problems of excessive sediment loading occur in lakes with large drainage basins where agricultural practices result in excessive soil erosion. Such problems are common in the central and southeastern parts of the United States, where row crop farming and erosive soils coexist, but some large reservoirs in the arid West also suffer from excessive sediment buildup. Siltation problems are significant in urban lakes as well. In extreme cases, excessive sedimentation leads to significant loss of reservoir storage capacity, diminishing the usefulness of lakes for regulating water availability (i.e., supplying water during droughts and controlling floods). Excessive sediment buildup renders large areas of lakes unusable for recreational purposes, as well as for fish spawning and habitat. Because nutrients (especially phosphorus) tend to adsorb onto sediments and because suspended sediments prevent penetration of light, lakes with very high loadings of sediment may not have sufficient plant productivity to support a good sport fishery; Lake Chicot, Arkansas, is an example (Stefan et al., 1990).
EXOTIC SPECIES
Lakes are island habitats. Like islands, they are highly susceptible to invasion by exotic species that lead to extirpation of native species

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(Magnuson, 1976). In some cases, invasions by exotic species have had severe environmental and economic consequences. The most notorious species invasions have widespread effects that reverberate throughout an ecosystem. The seemingly random nature and explosive development of biological invasions have fascinated ecologists for many years (Elton, 1958); the status of basic research on this topic was reviewed by Mooney and Drake (1986).
Many thousands of acres of inland lakes suffer from problems of excessive macrophyte growths, and in most cases the problem plants are exotic (nonnative) species. Some of these plants were introduced to this country by the aquarium industry; others, such as water hyacinth (Eichhornia crassipes), were imported because they were regarded as visually attractive. The natural predators and pathogens that tend to keep the plant populations in check in their native lands usually are not present in this country. The resulting uncontrolled growth causes a variety of problems: clogging of irrigation canals, hydro-electric systems, and navigational waterways; flooding due to obstructed drainage systems; and impairment of boating and contact recreational activities (Barrett, 1989). Cases have been reported of swimmers becoming entangled in excessive growths of macrophytes and drowning. Dense beds of plants alter water chemistry and habitat structure, leading to changes in invertebrate and fish communities, and they are a major source of organic matter to the water column and sediments. Some exotic plants (e.g., purple loosestrife and water hyacinth) have low nutritive value to aquatic animals and provide a poor base for the food chain. Aquatic weed invasions contributing to major management problems include water hyacinth in 50 countries on five continents, kariba weed (Salvinia molesta) in tropical regions worldwide, hydrilla (Hydrilla verticillata) and Eurasian water milfoil (Myriophyllum spicatum) in North America, and Elodea canadensis in Europe (Hutchinson, 1975; Barrett, 1989).
Exotic species problems are by no means limited to plants. Benthic invertebrate invaders also have created problems. An example is the invasion of lakes throughout northern Wisconsin and Minnesota by the rusty crayfish, Orconectes rusticus (Lodge et al., 1985). This species displaces native species from their burrows, exposing them to predation. Rusty crayfish are voracious consumers of game fish eggs and obliterate macrophyte beds, essential habitat for recruitment of game fish (Lodge et al., 1985). Thus, the crayfish tend to eliminate their main predators, smallmouth bass. Ironically, the invasion originated with releases from anglers' bait buckets. Spread of the crayfish is now perpetuated by the development of commercial harvesting of the rusty crayfish (primarily for export to Scandinavia). Crayfishers

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be restored. Paleolimnological approaches also should be used to infer whether a lake has been restored to its predisturbance condition.
Education and Training
The public needs to be better informed about the rationales, goals, and methods of aquatic ecosystem restoration. In addition, scientists with the broad training needed for aquatic ecosystem restoration are in short supply. The committee recommends the following:
Public education and outreach should be components of aquatic ecosystem restorations. Lake associations and citizen monitoring groups have proved helpful in educating the general public, and efforts should be made to ensure that such groups have accurate information about the causes of lake degradation and various lake restoration methods.
Funding is needed for both undergraduate and graduate programs in aquatic ecosystem restoration. Training programs must cross traditional disciplinary boundaries such as those between basic and applied ecology; between water quality management and fisheries or wildlife management; and among lake, stream, and wetlands ecology.
REFERENCES AND RECOMMENDED READING
Association of State and Interstate Water Pollution Control Administrators (ASIWPCA). 1984. America's Clean Water: The State's Evolution of Progress 1972–1982. ASIWPCA, Washington, D.C.
Association of State and Interstate Water Pollution Control Administrators (ASIWPCA). 1985. America's Clean Water: The States' Nonpoint Source Assesment. ASIWPCA, Washington, D.C.
Baker, L. A., and E. B. Swain. 1989. Review of lake management in Minnesota. Lake Reservoir Manage. 5:10–10.
Barrett, S.C.H. 1989. Waterweed invasions. Sci. Am. (October):90– 97.
Barten, J.M. 1987. Stormwater runoff treatment in a wetland filter: Effects on the water quality of Clear Lake. Lake Reservoir Manage. 3:297– 305.
Bauman, L. R., and R. A. Soltero. 1978. Limnological investigation of eutrophic Medical Lake, Wash. Northwest Sci. 52:127– 136.
Benndorf, J., and K. Putz. 1987. Control of eutrophication of lakes and reservoirs by means of pre-dams—I . Mode of operation and calculation of nutrient elimination capacity. Water Res. 21:829–838.
Benndorf, J., H. Schultz, A. Benndorf, R. Unger, E. Penz, H. Kneschke, K. Kossatz, R. Dumke, U. Hornig, R. Kruspe, and S. Reichel. 1988. Foodweb manipulation by enhancement of piscivorous fish stocks: Longterm